Ethernet.md

Ethernet

Each station has a NIC (Network Interface Card) that has a unique 6-byte MAC address.

How to handle collisions when channel is shared?

CSMA (Carrier Sense Multiple Access)

• Check if the channel is idle before sending
• If channel is busy, wait for a random time before retrying
• Can still have collisions when two stations start sending at the same time

CSMA/CD (Collision Detection)

• Detect collision by comparing the sent signal with the received signal
• If collision is detected, stop sending and wait for some time before retrying

Minimum Frame Size

• To ensure that the collision is detected before the frame is completely sent
• Should be 2 * Propagation Delay * Bandwidth

For 10 Mbps Ethernet, the minimum frame size is 512 bits (64 bytes), maximum transmission distance is 5km. Every frame less than 64 bytes is corrupted by the collision.

How long to wait before retrying? (Binary Exponential Backoff)

• After the k-th collision, wait for a random number of time units between 0 and 2^k - 1
• Each time unit equals to 2 * Propagation Delay, so that retry time is aligned
• The maximum value of k is 10
• After 16th collision, give up and report error

MAC Address

• 6 bytes, 48 bits

  • First 3 bytes: Organizationally Unique Identifier (OUI)
  • Last 3 bytes: NIC specific

• Multicast address: last bit of the first byte is 1

• Broadcast address: all bits are 1

Ethernet MAC Frame Format

	Destination MAC	Source MAC	Type	Data	FCS
Length	6 bytes	6 bytes	2 bytes	46-1500 bytes	4 bytes

• Type: 2 bytes, indicates the type of upper layer protocol
  • > 0x0600: Ethernet II (DIX V2)
  • <= 0x0600: IEEE 802.3, indicates the length of the data field
• Data: 46-1500 bytes, padding is added if the data is less than 46 bytes
• FCS (Frame Check Sequence): 4 bytes, CRC-32

Some typical type values:

• 0x0800: IPv4
• 0x0806: ARP
• 0x8100: VLAN
• 0x86DD: IPv6

In physical layer, 7 bytes of preamble and 1 byte of start frame delimiter (SFD) are added before the frame.